Various polyorganosiloxane compositions which are cured at room temperature to form elastomers are known.
For example, compositions comprising a polydiorganosiloxane having both terminals end-blocked with silanol groups, and a crosslinking agent comprised of an organic silicon compound having a hydrolyzable group bonded to the silicon atom, such as an acetoxy group, an alkoxy group, a dialkylketoxime group, a dialkylamino group, a dialkylaminoxy group or an N-methylamido group, are widely used in the construction industry, the automobile industry, the electric machinery industry and other industries. In the construction industry, these compositions are used for forming waterproof seals in spaces of walling materials such as concrete walls and aluminum plates, fixing glass sheets to sashes, and bonding glass sheets to one another. Such compositions are generally called silicone sealants. They exhibit excellent weatherability, durability, heat resistance and cold resistance, and experience only small changes in physical properties as the temperature changes, and they are not degraded by ozone or ultraviolet rays. Moreover, they have good workability in the application operation. By virtue of these characteristic properties, market demands for these silicone sealants have prominently increased in the field of construction of multi-storied buildings as well as in other fields. Moreover, these compositions have been recently used as coating materials. More specifically, a crosslinkable polyorganosiloxane of this type may be coated on a roof or wall surface in order to form a silicone elastomer layer.
Although such silicone elastomers have the above-mentioned excellent properties, they still retain a certain stickiness upon completion of the curing reaction. Furthermore, since the polydiorganosiloxane, which is the main component, is a dielectric substance, the silicone elastomer tends to attract dust. but the elastomer surface is markedly stained by the attracted dust, especially when a non-reactive polydiorganosiloxane is also added so as to obtain a silicone elastomer having suitable softness while retaining suitable operation workability. Furthermore, since the polysiloxanes which do not participate in the crosslinking reaction may tend to ooze out to the surface and contact the attracted dust, and the dust is rendered water-repellant, and will therefore not be washed away by rain water. Thus, the appearance may be drastically degraded by such dust.
We have previously found that a method of making a surface active agent co-present in a polyorganosiloxane composition that can be cured into an elastomer which is effective for preventing the above-described contamination (Japanese Patent Application No. 153148/79). This method is advantageous since a contamination-preventing agent is incorporated in a sealant and, the number of working steps need not be increased. Even when the joint width considerably changes, sealing can be effected appropriately despite such change of the joint width. However, the mechanical properties and adhesion of the sealant are reduced when it is impregnated with water. In order to eliminate this disadvantage, it is necessary to use an inorganic filler having a hydrophobic surface.
We then found that a dust-resistant film is effectively formed by a method comprising coating on the surface of a silicone elastomer such as mentioned above, a composition formed by dissolving a benzene-soluble polyorganosiloxane resin containing trifunctional or tetrafunctional siloxane units in which the molar ratio of the organic group to the silicon atom in the molecule is lower than 2, into a mixed solvent of a hydrocarbon solvent and a volatile silane or siloxane, and drying and curing the coated composition (Japanese Patent Application No. 173443/79). This method is advantageous in that a uniform film having high dust resistance can be formed on the surface of a silicone elastomer. However, the film obtained according to this method has unsatisfactory softness and toughness. Accordingly, if the sealing composition is applied to a place where stress is easily caused by vibrations of a base or expansion or shrinkage of the base by changes of temperatures, for example, if the composition is used for sealing joints of a multi-storied building or coating the surface of a construction, cracks are readily formed in the film and the appearance is drastically degraded by contamination of the cracked portions.
Research was directed toward developing a method capable of solving the above-mentioned problems and we have completed the present invention which addresses these problems.